Wavelength division multiplexing (WDM) is one technique used to increase the capacity of optical transmission systems. A wavelength division multiplexed optical transmission system employs plural optical channels, each channel being assigned a particular channel wavelength. In a WDM system, optical channels are generated, multiplexed to form an optical signal comprised of the individual optical channels, transmitted over a waveguide, and demultiplexed such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of wavelength division multiplexing in long-distance optical systems. Some WDM systems currently under development will have thirty or more closely spaced channels separated by a spacing on the order of 0.5 to 5 nm and are referred to as Dense Wavelength Division Multiplexing (DWDM) systems. In connection with the present invention, the terms WDM and DWDM will often be used interchangeably herein.
WDM systems have been deployed in long distance networks in a point-to-point configuration consisting of end terminals spaced from each other by one or more segments of optical fiber. In metropolitan areas, however, WDM systems having a ring or loop configuration are currently being developed. Such systems typically include a plurality of nodes located along the ring. At least one optical add/drop element, associated with each node, is typically connected to the ring with optical connectors. The optical add/drop element permits both addition and extraction of channels to and from the ring. A particular node that allows the addition and extraction of all the channels is commonly referred to as a hub or central office node, and typically has a plurality of associated add/drop elements for transmitting and receiving a corresponding plurality of channels to/from other nodes along the ring.
Optical signals in WDM networks experience degradations (i.e., degradations in the optical signal-to-noise ratio) due to ASE (Amplified Spontaneous Emission) accumulation as well as effects such as PMD (polarization Mode Dispersion), PDL (Polarization Dependent Loss), dispersion and fiber non-linearities, which arise as the signals propagate through the optical network. As a consequence, optical signals sometimes require regeneration so that they can be transmitted over extended distances. The regeneration can involve processes in the optical domain, such as optical amplification, dispersion compensation and PMD compensation. The regeneration can also involve additional processes. In addition to these optical processes, regeneration can also involve re-shaping and re-timing of individual wavelength channels, which typically is achieved by converting the wavelength channel into the electrical domain, and back into the optical domain. In addition to regenerating the signal, a regenerator can also perform wavelength conversion so that the output signal of the regenerator is emitted at a wavelength different from the wavelength of the input signal. Regenerators incorporating wavelength conversion not only extend the distance a signal can propagate in the network, but also serve to avoid wavelength contention, thereby increasing the effective capacity of the network. In a conventional arrangement regeneration is accomplished by converting the wavelength channels into the electrical domain and back into the optical domain (a so-called opto-electronic conversion). Regeneration can also be accomplished by all optical means (a so-called all-optical regeneration), although this is not widely used in today's network.
In current networks regeneration is performed at preselected nodes. For example, in a network having a ring topology regeneration is typically performed at hub-nodes that inter-connect individual rings. The hub-nodes typically regenerate each and every one of the wavelength channels, regardless of whether the channels actually require regeneration or not. That is, the hub-node contains a regenerator for each and every wavelength employed in the network. Accordingly, the hub-node must include more regenerators than are absolutely necessary to regenerate only those channels in need of regeneration. For example, if a particular wavelength channel originates at a node close to the hub-node, it is unlikely to require regeneration as it traverses the hub-node. Nevertheless, the channel would undergo regeneration in the hub-node, thus leading to higher than necessary overall network costs.
It should be noted that the previously mentioned considerations are applicable to a ring network having a static traffic pattern. In the more general case of a ring network having a non-static traffic pattern, each node should be equipped with a regenerator for all wavelength channels entering that node, further increasing overall network costs. Moreover, these considerations are equally applicable to other network topologies such as a mesh topology. In contrast to a ring topology, the channels in a mesh network can take any path from its origination node to its destination node. As a result, like in a ring network with non-static traffic patterns, all nodes must be able to regenerate each and every wavelength employed in the network to ensure that they can all be transmitted successfully from any origination node to any destination node. It is very cost ineffective to make all nodes in a network capable of regenerating all traffic passing through that node.
Accordingly, it would be advantageous if the number of regenerators employed in an optical transmission system could be reduced while still ensuring the successful transmission of all wavelength channels in the network.